Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products comprising olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces some molecules that tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that can foul equipment under certain conditions. In general, feedstocks containing higher boiling materials tend to produce greater quantities of tar.
Olefin gas cracker systems are normally designed to crack ethane, propane and on occasion butane, but typically lack the flexibility to crack heavier feedstocks, such as liquids particularly those feedstocks that produce tar in amounts greater than one percent. As gas feeds tend to produce little tar, primary, secondary, and even tertiary transfer line exchangers (TLEs) are utilized to recover energy through the generation of high pressure and medium pressure steam, as the furnace effluent cools from the furnace outlet to the quench tower inlet. TLE fouling on the process side is very limited with gas feeds, since the tar yields are very low.
The process gas is normally then fed to a quench tower wherein the process gas is further cooled by direct contact with quench water. Typically, the bottoms of the quench tower feed a quench drum, which functions as a three-phase separator, with a light hydrocarbon phase that floats on water and tar, which sinks in water, as the bottom phase. Even in the case of cracking ethane feed, the tar yield is high enough to cause the water leaving the quench drum to contain enough light tar, which has a specific gravity close to that of water, to cause downstream fouling of the quench circuit. This can result in the fouling of downstream heat exchangers and water stripping towers, which, when fouled, must be taken offline for cleaning.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks which contain a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, by way of non-limiting examples, atmospheric residue, e.g., atmospheric pipe still bottoms and crude oil. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). The non-volatile components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized.
Additionally, during transport, some naphthas or other lighter liquids are contaminated with heavy crude oil containing non-volatile components. Conventional pyrolysis furnaces do not have the flexibility to process residues, crudes, or many residue or crude contaminated gas oils or naphthas which comprise non-volatile components.
As indicated, in most commercial naphtha crackers, cooling of the effluent from the cracking furnace is normally achieved using a system of transfer line heat exchangers, a primary fractionator and a water quench tower or indirect condenser. The steam generated in transfer line exchangers can be used to drive large steam turbines which power the major compressors used elsewhere in the ethylene production unit. To obtain high energy-efficiency and power production in the steam turbines, it is necessary to superheat the steam produced in the transfer line exchangers.
Cracking heavier feeds, such as kerosenes and gas oils, may produce large amounts of tar, which can lead to rapid coking in the radiant section of the furnace as well as fouling in the transfer line exchangers preferred in lighter liquid cracking service, often requiring costly shutdowns for cleaning. Furthermore, if a quench liquid such as water is used, the heavy oils and tars may form stable emulsions that make it difficult to dispose of excess quench water in an environmentally acceptable manner.
As indicated above, one technique used to further quench the effluent produced by steam cracking and remove the resulting heavy oils and tars employs a water quench tower in which the condensables are removed at near ambient conditions. Such a water quench technique has proven acceptable when cracking light gases, primarily ethane, although the quench water still may have significant amounts of hydrocarbons present, which serve to foul the water quench circuit. An alternative and more complex technique utilizes an oil quench with fractionation to remove the heavier tars, followed by a water quench to remove other condensables and complete the cooling. This technique is most practical for naphtha or heavy oil crackers which produce from about 1.0 wt. % tar to greater than about 30 wt. % tar.
Neither of these techniques is, however, entirely optimum for use in steam crackers that crack liquefied petroleum gases, light naphthas, and ethane that produce relatively little heavy oil and tar. One issue with these feedstocks stems from the fact that some of the heavy oils and tars produced when the pyrolysis effluent of these feedstocks is quenched have approximately the same density as water and can form stable oil/water emulsions. Emulsion formation can render water quench operations ineffective, causing dilution steam generators to foul, and make disposal of excess quench water in an environmentally acceptable manner difficult. Moreover, this further complicates the disposal of heavy oil and tar.
Alternatively, a primary fractionator would prevent the formation of oil/water emulsions by removing the heavy oils and tars in the primary oil quench stage. Such a system could, however, be more costly to construct and operate than a simple water quench system. Additionally, the primary fractionator system may not generate sufficient heavy oil to allow it to replenish its own quench oil, some of which must be continuously removed to dispose of accumulated tars. As such, operation of a primary fractionator under these conditions would require the added expense of an external supply of quench oil. Furthermore, logistical difficulties are presented if the cracker is not located adjacent to a facility capable of providing quench oil and removing spent oil.
Steam crackers designed to operate on gaseous feedstocks, while limited in feedstock flexibility, require significantly lower investment when compared to liquid feed crackers designed for naphtha and/or heavy feedstocks that produce higher amounts of tar and byproducts. However, as may be appreciated, when the price of natural gas is high relative to crude, gas cracking tends to be disadvantaged when compared with the cracking of virgin crudes and/or condensates, or the distilled liquid products from those feeds. (e.g., naphtha, kerosene, field natural gasoline, etc). In such an economic environment, it would be desirable to extend the range of useful feedstocks to include liquid feedstocks that yield higher levels of tar. Therefore, there is a need for an improved method of quenching effluent and removing the resulting heavy oils and tars.